Body Mass Index (BMI)


Body Mass Index is a measure that uses your height and weight to calculate if you are at a healthy weight. The BMI calculation divides an adult’s weight in kilograms by their height in metres squared.

BMI Explained

Rosemary asks fitness supremo Mary Morris to explain in simple terms the implications of our BMI and why being more active as we get older is beneficial to our weight and long term happiness.

Body Mass Index Range

For most adults, an ideal BMI is in the 18.5 to 24.9 range.

If your BMI is:

  • below 18.5 – you’re in the underweight range
  • between 18.5 and 24.9 – you’re in the healthy weight range
  • between 25 and 29.9 – you’re in the overweight range
  • 30 or over – you’re in the obese range

BMI takes into account natural variations in body shape, giving a healthy weight range for a particular height.

As well as measuring your BMI, healthcare professionals may take other factors into account when assessing if you’re a healthy weight. Muscle is much denser than fat, so very muscular people, such as heavyweight boxers, weight trainers and athletes, may be a healthy weight even though their BMI is classed as obese. For children and young people aged 2 to 18, a BMI calculation conducted by a health professional should take into account age and gender as well as height and weight.

For older people aged 65 years+ much research has been done looking at what point our weight is likely to cause severe illness and death and there is now some adjustment to the ranges:

If your BMI is:

  • below 23 – you’re in the underweight range
  • between 23 and 29.9 – you’re in the healthy weight range
  • 30 or over – you’re in the overweight/obese range

Meet Mary Morris MSc.

Rosemary asks Mary Morris about her distinguished career in the fitness industry, her life and what drove her to become such a respected fitness expert and Teacher Trainer.